Re: [alsa-devel] Using snd_pcm_set_params() on a capture handle

I've got a simple PCM capture test program below, which is basically a slightly tweak version of something from:

http://equalarea.com/paul/alsa-audio.html

The program uses two different methods of setting up the capture handle; one that works (and is complex) and one that doesn't but seems like it should (and is simple). I'm trying to figure out why the simple one (using snd_pcm_set_params) doesn't work.

The program can be compiled and run in two ways:

gcc -Wall -O2 -DUSE_SET_PARAMS=1 alsa_capture.c -lasound -o

alsa_capture

./alsa_capture

which fails with:

Read from audio interface failed (Input/output error)

and:

gcc -Wall -O2 -DUSE_SET_PARAMS=0 alsa_capture.c -lasound -o

alsa_capture

./alsa_capture

which works as expected.

Questions:

• Is snd_pcm_set_params() supposed to work on capture handles (I've used it successfully on playback handles)?

• Is my use of snd_pcm_set_params() missing something and if so what?

Do your sound card driver support 0.5 second buffer?

• If snd_pcm_set_params() is not supposed to work on capture handles would it be possible to update the documentation to reflect that?

    int buffer_frames = 128;
    unsigned int rate = 44100;
    snd_pcm_t *capture_handle;
    snd_pcm_format_t format = SND_PCM_FORMAT_S32_LE;
  

    if (USE_SET_PARAMS) {
            if ((err = snd_pcm_set_params (capture_handle, format,

SND_PCM_ACCESS_RW_INTERLEAVED, 2, rate, 0, 500000)) < 0) { /* 0.5sec */

                    printf ("Capture open error: %s\n", snd_strerror

(err)); exit (1) ;

            }
    }
    else {
            snd_pcm_hw_params_t *hw_params;

            if ((err = snd_pcm_hw_params_malloc (&hw_params)) < 0) {
                    printf ("Cannot allocate hardware parameter

structure (%s)\n", snd_strerror (err)); exit (1) ;

            }

            if ((err = snd_pcm_hw_params_any (capture_handle,

hw_params)) < 0) {

                    printf ("Cannot initialize hardware parameter

structure (%s)\n", snd_strerror (err)); exit (1) ;

            }

            if ((err = snd_pcm_hw_params_set_access (capture_handle,

hw_params, SND_PCM_ACCESS_RW_INTERLEAVED)) < 0) {

                    printf ("cannot set access type (%s)\n",

snd_strerror (err)); exit (1) ;

            }

            if ((err = snd_pcm_hw_params_set_format (capture_handle,

hw_params, format)) < 0) {

                    printf ("Cannot set sample format (%s)\n",

snd_strerror (err)); exit (1) ;

            }

            if ((err = snd_pcm_hw_params_set_rate_near

(capture_handle, hw_params, &rate, 0)) < 0) {

                    printf ("Cannot set sample rate (%s)\n",

snd_strerror (err)); exit (1) ;

            }

            if ((err = snd_pcm_hw_params_set_channels

(capture_handle, hw_params, 2)) < 0) {

                    printf ("cannot set channel count (%s)\n",

snd_strerror (err)); exit (1) ;

            }

            if ((err = snd_pcm_hw_params (capture_handle, hw_params))

< 0) {

                    printf ("cannot set parameters (%s)\n",

snd_strerror (err)); exit (1) ;

            }

If you does not specify period, period_size or buffer size, period_time or buffer_time before calling snd_pcm_hw_params , you have to use use those hw_params_get_* functions after calling snd_pcm_hw_params to get back those values

    buffer = malloc (buffer_frames * snd_pcm_format_width(format) / 8

* 2);

    if ((err = snd_pcm_prepare (capture_handle)) < 0) {
            printf ("cannot prepare audio interface for use (%s)\n",

snd_strerror (err)); exit (1) ;

    }

    for (i = 0; i < 10; ++i) {
            if ((err = snd_pcm_readi (capture_handle, buffer,

buffer_frames)) != buffer_frames) {

                    printf ("Read from audio interface failed

(%s)\n", snd_strerror (err)); exit (1) ;

                    break;
                    }
            printf("read %d done\n", i);
    }